On-Screen Ruler - lightweight, free, semi-transparent ruler
Meazure - What are the dimensions of that image? How big is this window?
JR Screen Ruler - JR Screen Ruler, also known as JRuler - Free virtual ruler for your computer screen.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
PixelStick - PixelStick is a tool for measuring distances and angles on the screen.
PMeter - PMeter is a very handy pixel meter.